The incident diameter of a materials processing laser beam is one of the most important parameters in a laser-materials processing system. The specific diameter of interest is that of the laser beam at a point of laser beam-material interaction on a workpiece being subjected to laser processing. Such processing is that which is typically performed with a high power (approximately 50 watts or more) materials processing laser and can include cutting, drilling or welding operations the workpiece.
The incident beam diameter, among other laser-materials process parameters such as the speed at which the beam traverses the workpiece and beam power, is critical to controlling the nature and quality of the processing operation being performed. That is, it is critical to know the incident beam diameter and control it to assure a particular process result. It is currently known in the art to estimate, as opposed to directly measure, the incident beam diameter by using geometrical optics or by inspecting the workpiece after processing. Measuring melt widths on the workpiece after processing provides some indication of beam diameter. The use of melt isotherms for determining incident beam diameter is also known but lacks accuracy since the isotherms typically lie outside the actual beam diameter. These and other estimating techniques typically provide beam diameter with limited accuracy and consistency and, in some cases, by their nature are only performable after the processing is complete.
It would therefore be desirable to provide apparatus and method for directly measuring, rather than merely estimating, the incident diameter of a materials processing laser beam. Further, it would be highly desirable for such beam diameter determination apparatus and method to provide determined beam diameter information in real time so that the laser-materials process can be controlled, in real time, in accordance therewith. In the broadcast sense, real time information and control are used herein to describe information related to the process, such as incident laser beam diameter, obtained during process operation and the control of process parameters in accordance with the real time information, such control also being effected during process operation. It is noted that the time period between obtaining real time information and the responsive adjustments of process parameters will vary depending on the nature of the particular process.
One technique known in the art for directly measuring beam diameter is to make burn patterns on photosensitive paper (typically for low paper applications) or on a Lucite.RTM. block (typically for CO.sub.2 lasers). The burn pattern technique is not a per se real-time measurement of beam diameter since the pattern must be evaluated by a human system operator in order to determine the diameter. Further, unless the burn pattern material directly overlays the workpiece, a practice which may interfere with the operation of a robotic vision control feedback system, the beam must be directed away from the workpiece in order to perform the diameter determination. The use of a photo detector array enables a second technique, known in the art, for direct measurement of beam diameter. The laser beam is directed onto the photo detector array and the responsive signals provided by the photo detector elements comprising the array enable, inter alia, beam diameter measurements. Such arrays, as known to the inventors herein are, however, limited to use with low power lasers (e.g. a maximum laser power on the order of 0.5 watts) and are therefore not suitable for application in the laser-materials processing system contemplated herein. Further, as in the case of the burn pattern technique, the laser beam must be directed away from the workpiece in order to perform the beam diameter determination. As a result, the diameter determination is not truly performed in real time. One photo detector array enabling beam diameter measurement, as well as software for the processing of the photo detector array signals, is manufactured by Spiricon, Inc. of Logan, Utah as the Laser Matrix Probe.
Other software packages are known in the art that process a digitized image, such as derived from video camera apparatus, of a laser beam. Such processing is adapted to perform an extensive set of beam diagnostic functions. For example, the BEAMCODE diagnostic system sold by Big Sky Software Corporation of Bozeman, Mont. processes beam image information and provides graphical and/or text representations of beam characteristics including intensity profile, diameter, power, divergence, centroid, peak and three-dimensional isometric representation. Such software packages, however, are not intended for and cannot be implemented for the real time process control application contemplated herein. They are instead adapted for "off-line" processing of the beam image data. Due to the extensive number of complex diagnostic functions performed, a substantial amount of computing time is required for the ultimate provision of relevant beam characteristics. This substantial computing time renders the performance of such software packages too slow for the real time application contemplated herein. Further, the digital representation of the information relevant to the present control application, i.e. the beam diameter, is part of an overall information readout in such software packages and cannot be isolated for direct use in process control. That is, no discrete signal proportional to beam diameter is provided to enable process control in accordance therewith.
It is therefore a principal object of the present invention to provide apparatus and method for direct measurement of the incident beam diameter of a materials processing laser beam.
It is a further object of the present invention to provide apparatus and method for direct, real-time determination of incident beam diameter of a materials processing laser in a laser-materials processing system.
It is an additional object of the present invention to provide a laser-materials processing system including apparatus for direct, real-time determination of incident laser beam diameter and control of the process in accordance with the determined beam diameter.